Fix sexpr2string() to handle empty list.
Fix sexpr2string() to handle empty list.

S-expression containing empty lists, e.g. (cpus (() () () ())),
was not being handled properly in sexpr2string() serialization.
Emit an empty list when encountering NIL sexpr kind.

shugaoye/libvirt,jardasgit/libvirt,danwent/libvirt-ovs,taget/libvirt,rlaager/libvirt,usc-isi/libvirt,jardasgit/libvirt,dumbbell/libvirt,jeckersb/libvirt,crobinso/libvirt,novel/fbsd-libvirt,VenkatDatta/libvirt,datto/libvirt,iam-TJ/libvirt,jfehlig/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,sshah-solarflare/Libvirt-PCI-passthrough-,wiedi/libvirt,crobinso/libvirt,leilihh/libvirt,danwent/libvirt-ovs,trainstack/libvirt,dumbbell/libvirt,cbosdo/libvirt,zippy2/libvirt,wiedi/libvirt,olafhering/libvirt,novel/fbsd-libvirt,VenkatDatta/libvirt,eskultety/libvirt,amery/libvirt-vserver,rmarwaha/libvirt1,datto/libvirt,zippy2/libvirt,bjzhang/libvirt,iam-TJ/libvirt,amery/libvirt-vserver,emaste/libvirt,kantai/libvirt-vfork,cbosdo/libvirt,jeckersb/libvirt,soulxu/libvirt-xuhj,zhlcindy/libvirt-1.1.4-maintain,fabianfreyer/libvirt,zhlcindy/libvirt-1.1.4-maintain,fabianfreyer/libvirt,usc-isi/libvirt,zhlcindy/libvirt-1.1.4-maintain,rmarwaha/libvirt,siboulet/libvirt-openvz,elmarco/libvirt,leilihh/libvirt,bjzhang/libvirt,elmarco/libvirt,rmarwaha/libvirt1,wiedi/libvirt,leilihh/libvirt,zhlcindy/libvirt-1.1.4-maintain,danwent/libvirt-ovs,olafhering/libvirt,eskultety/libvirt,elmarco/libvirt,trainstack/libvirt,trainstack/libvirt,rbu/libvirt,amery/libvirt-vserver,taget/libvirt,siboulet/libvirt-openvz,agx/libvirt,usc-isi/libvirt,warewolf/libvirt,datto/libvirt,andreabolognani/libvirt,eskultety/libvirt,novel/fbsd-libvirt,soulxu/libvirt-xuhj,wiedi/libvirt,wiedi/libvirt,jeckersb/libvirt,rlaager/libvirt,siboulet/libvirt-openvz,agx/libvirt,zippy2/libvirt,rmarwaha/libvirt,wiedi/libvirt,libvirt/libvirt,amery/libvirt-vserver,rmarwaha/libvirt1,nertpinx/libvirt,rmarwaha/libvirt,agx/libvirt,emaste/libvirt,rmarwaha/libvirt1,jfehlig/libvirt,libvirt/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,shugaoye/libvirt,eskultety/libvirt,zhlcindy/libvirt-1.1.4-maintain,warewolf/libvirt,libvirt/libvirt,olafhering/libvirt,rbu/libvirt,agx/libvirt,trainstack/libvirt,agx/libvirt,iam-TJ/libvirt,iam-TJ/libvirt,foomango/libvirt,jfehlig/libvirt,andreabolognani/libvirt,novel/fbsd-libvirt,emaste/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,warewolf/libvirt,novel/fbsd-libvirt,novel/fbsd-libvirt,siboulet/libvirt-openvz,VenkatDatta/libvirt,danwent/libvirt-ovs,elmarco/libvirt,kantai/libvirt-vfork,sshah-solarflare/Libvirt-PCI-passthrough-,warewolf/libvirt,iam-TJ/libvirt,leilihh/libvirt,rlaager/libvirt,shugaoye/libvirt,olafhering/libvirt,taget/libvirt,leilihh/libvirt,trainstack/libvirt,cbosdo/libvirt,elmarco/libvirt,shugaoye/libvirt,dumbbell/libvirt,VenkatDatta/libvirt,wiedi/libvirt,kantai/libvirt-vfork,bjzhang/libvirt,shugaoye/libvirt,usc-isi/libvirt,fabianfreyer/libvirt,foomango/libvirt,kantai/libvirt-vfork,cbosdo/libvirt,datto/libvirt,taget/libvirt,iam-TJ/libvirt,taget/libvirt,warewolf/libvirt,crobinso/libvirt,nertpinx/libvirt,nertpinx/libvirt,zippy2/libvirt,foomango/libvirt,siboulet/libvirt-openvz,emaste/libvirt,jeckersb/libvirt,rmarwaha/libvirt1,jardasgit/libvirt,bjzhang/libvirt,andreabolognani/libvirt,rmarwaha/libvirt,jfehlig/libvirt,crobinso/libvirt,rbu/libvirt,novel/fbsd-libvirt,novel/fbsd-libvirt,jardasgit/libvirt,dumbbell/libvirt,soulxu/libvirt-xuhj,warewolf/libvirt,jeckersb/libvirt,leilihh/libvirt,rbu/libvirt,dumbbell/libvirt,trainstack/libvirt,emaste/libvirt,novel/fbsd-libvirt,kantai/libvirt-vfork,eskultety/libvirt,warewolf/libvirt,soulxu/libvirt-xuhj,sshah-solarflare/Libvirt-PCI-passthrough-,bjzhang/libvirt,trainstack/libvirt,jardasgit/libvirt,datto/libvirt,rmarwaha/libvirt1,rbu/libvirt,nertpinx/libvirt,cbosdo/libvirt,dumbbell/libvirt,rlaager/libvirt,jeckersb/libvirt,soulxu/libvirt-xuhj,usc-isi/libvirt,jeckersb/libvirt,rmarwaha/libvirt,emaste/libvirt,amery/libvirt-vserver,iam-TJ/libvirt,foomango/libvirt,danwent/libvirt-ovs,libvirt/libvirt,rlaager/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,rmarwaha/libvirt,nertpinx/libvirt,emaste/libvirt,foomango/libvirt